For (rapid) digital data transmission over short distances, that is to say distances of a few meters or centimeters, conductors are often used in a parallel arrangement and in countless variants.
One application concerns, for example, parallel buses (data transmission paths) on motherboards of personal computers (PC), to which a plurality of daughter boards can be connected in a parallel manner. Such buses usually have a length of less than 300 mm and may have more than 100 parallel conductors, for example ISA bus, PC104 bus, PCI bus, or else numerous proprietary buses without compulsory standards.
Another example concerns data transmission over distances of several meters. Multicore screened cables are used in the prior art for rapid parallel data transport between devices such as PCs and printers or measuring devices. Typical examples are: the PC parallel port, Centronics/IEEE-1284, the IEEE-488/IEC-625 instrument bus or else all industrial control systems with multicore ribbon cables.
The common feature of these parallel data transmission operations is the high degree of complexity in the transmitting and receiving sections and the large number of parallel lines needed for the transmission cable. Typical characteristics of parallel conductors are their considerable susceptibility to faults as a result of electromagnetic fields, crosstalk and propagation time differences between the parallel conductors. The inflexibility in terms of the degree of expansion relating to the data bus width, the address range and the transmission rate is particularly troublesome in some cases.
For these reasons, serial data transmission was widely used. Virtually only the serial principle is possible in many applications during wireless transmission via radio or light waves or else during wired transmission via telecommunication lines.
Many serial data transmission protocols, such as PCI-Express, ETHERNET, EtherCAT, Powerlink, USB, or industrial field buses, for instance Profibus, Device-Net or CANopen, have become established in the respective fields of application and have been published in corresponding standards.
Serial data transmission has simplified and reduced the cost of digital data traffic. Different coding and checking methods guarantee secure and robust links. The possibility of some coding methods, for example also the Manchester coding method mentioned below, extracting the clock signal from the data stream solves the propagation time problems over any desired transmission paths and distances.
A high degree of flexibility for the number of bus subscribers, the data width, the address ranges, the transmission media, the transmission distance and the transmission direction is provided for many of these methods.
However, a particular problem arises when using serial digital data transmission operations over short distances in machine tools and similar electronic systems if randomly occurring events have to be handled in arbitrary subassemblies of the machine (virtually) without a time delay.
Although solutions, for instance EtherCat and Powerlink, are known for cyclically synchronizing different modules, for example drive shafts, with one another, no usable approaches are known for the stochastic operations.
The European patent specification EP 1 749 609 B1 discloses consistent modularization of the subassemblies in machine tools, in particular in electrical discharge machines. Starting from a central communication node, the modules are networked in a star configuration via ETHERNET-like data links and are also supplied with DC voltage via the data cables up to a power of 50 W. Power is supplied, for example, according to the Power over ETHERNET standard, or PoE for short, which is similar to the IEEE802.3af standard.
The communication node additionally has at least one standard ETHERNET data link according to IEEE802.3 which allows any desired long-distance links to the outside world.
Such a machine tool in the end no longer requires an electrical cabinet and can be expanded or modified at any time. Remote diagnoses, configurations and software updates can be conveniently carried out via the Internet. The modules are directly installed at their place of action in the machine tool in order to keep losses produced when transmitting power as low as possible.
Three priorities for data processing and data transmission are proposed in EP 1 749 609 B1 as further information for configuring the internal, ETHERNET-like links LINK:                TOP priority: parallel processing only inside a module,        SECOND priority: between modules, via the network node and preferably parallel data processing,        THIRD priority: sequential data processing is preferred between modules and the network node and to external, superordinate systems.        
As mentioned at the outset, ETHERNET derivatives such as EtherCAT, Powerlink, Profinet and Modbus TCP/IP have furthermore been disclosed for real-time applications. These involve particular precautions in the transmitting method in order to guarantee a maximum permissible latency for a deterministic (predictable) behaviour. For this purpose, a time stamp according to IEEE1588 is concomitantly transmitted in the data packets, for example, in order to allow precise temporal synchronization of a plurality of bus subscribers.
However, all of these known solutions have the common disadvantage that they cannot immediately react to temporally random operations. The transmission of a data packet must always be initiated or awaited in order to transmit an item of information, and the event must also be synchronized with the local clock rates in each case.
For an EtherCAT link with a Cat. 5 cable according to the EIA/TIA-568 standard and the maximum bandwidth of 100 MHz for example, this now results in a time delay of at least 10 μs and considerable jitter (temporal unsharpness). Since a sensor and an actuator are usually involved in the operation and likewise result in a certain time delay in the data processing, this time delay before it is possible to react to a stochastic event may double to more than 20 μs, for example.
Although the faster Cat. 6 cables according to the EN50288 standard, with a maximum bandwidth of 250 MHz, or Cat. 6a links at 500 MHz for Gigabit Ethernet according to IEEE802.3an could reduce this time delay before reacting to a stochastic event, they increase the system costs and the power losses in doing so.
Even with a bandwidth of 500 MHz and parallel transmission via four conductor pairs, delay times before reacting to a random event of approximately 2 μs could still be expected for the sensor/actuator case.
As soon as random operations or events which typically have to be processed in less than approximately 100 ns occur in a system, the synchronous, serial data transmission operations described above are generally overtaxed, for example.
Electrical discharge machines are particularly affected by this disadvantage since many process-related operations have a purely stochastic behaviour, for instance the ignition delay time of the processing pulses or process faults which should be detected during a processing pulse and should be eliminated by immediate measures. Some electrical discharge processes are based on weak, so-called preliminary or probing pulses which scan the state of the spark gap. Depending on the analysis result, a processing pulse or various other pulses, for instance for breaking open micro-short circuits or for cleaning via shockwaves, is/are released. However, this release must be carried out in less than approximately 100 ns since otherwise the method becomes futile because the measured properties of the spark gap are already no longer current (for example after 100 ns).
Similar problems occur in highly dynamic servo shafts with linear motors or piezo drives. Such systems are being used more and more often to stabilize processes after continuously detected, stochastic interference variables. Examples are: drive shafts in electrical discharge machines, the out-of-round turning in turning machines, the active damping of external vibrations in high-precision machines and the active suppression of chatter marks caused by the vibrations of cutting tools in cutting machine tools.
Actuating speeds of up to 10 m/s are not uncommon for this type of drive. However, this means that, in this case, the position changes by 1 μm in 100 ns. For micrometer accuracy, a limit of approximately 100 ns thus results for the delay time before taking countermeasures when stochastic events occur.
Errors of up to 20 μm should accordingly be expected for a delay time of 2 μs which, as mentioned above, would occur in the sensor/actuator case.
In comparison with the prior art, the invention is based on the object of developing a serial method and an apparatus for digital data transmission in such a manner that stochastic events can be analysed with little effort and little energy consumption and can be rapidly transmitted with little energy consumption.